1. Field of the Invention
The present invention relates to a mobile communication system, such as a portable telephone system, and in particular, to an amplifying device for use in a base station device.
2. Description of the Related Art
In the mobile communication system, such as the portable telephone system, a base station device is required to ensure arrival of a radio signal at a mobile station device which is located far from the base station device. Accordingly, it is necessary for the base station device to largely amplify a signal through an amplifier for transmitting it.
In the foregoing mobile communication system, the W-CDMA (Wide-band Code Division Multiple Access) system and so on have been adopted as mobile communication systems.
However, since an amplifier is an analog device, its input-output characteristics form a nonlinear function. Particularly, after exceeding the amplification limit point called a saturation point, the output power remains nearly constant even if the power inputted to the amplifier increases.
This nonlinear output causes nonlinear distortion. In case of a transmission signal before amplification, a signal component outside a desired signal band is suppressed to a low level through a band limiting filter. On the other hand, in case of a signal after passing the amplifier, nonlinear distortion is generated so that a signal component leaks to the exterior of the desired signal band (adjacent channels). This causes a phenomenon that the power spectrum expands to the adjacent channels.
As described above, since the transmission power is high in the base station device, the magnitude of the leak power to the adjacent channels is strictly regulated. Under these circumstances, it has been a large problem how to reduce the foregoing adjacent channel leak power.
For amplifying the transmission power while reducing the adjacent channel leak power, a distortion compensation amplifying device using the predistortion technique has been provided in the base station device for amplifying the transmission power.
The conventional distortion compensation amplifying device using the predistortion technique is described in JP-A-2000-151295 for “DISTORTION COMPENSATION CIRCUIT” published on May 30, 2000 (Applicant: Mitsubishi Electric Corporation; Inventors: Kenichi Horiguchi and others).
In this prior art, a portion of an input signal inputted to a signal path having a vector adjuster, a linearizer and an amplifier and a portion of an output signal outputted from the amplifier are respectively extracted, and a combined power level of these signals is detected, and then, based on a detection result, a bias adjustment of the linearizer and an adjustment of the vector adjuster are carried out so as to minimize the power level.
However, since the foregoing conventional amplifying device uses the analog delay line to delay the input signal, it is increased in size, weight and cost. In view of this, it has been under review to achieve an amplifying device without using the analog delay line, wherein a digital delay line is used instead of the analog delay line, the power detection is carried out using a digital input signal, and phases of the multi-carrier signal and the power value signal are synchronized.
In the distortion compensation amplifying device using the predistortion technique, it is necessary to match the phases of the multi-carrier signal and the power value signal in the predistorter for giving accurately the distortion compensation characteristic to the input signal. For detecting the power value using the digital input signal as referred to above, it is necessary to perform a frequency adjustment in consideration of frequency differences between the respective carriers. By matching phases of this processing and the conversion to the carrier frequencies with each other, the phases of the multi-carrier signal and the power value signal can be matched.
However, while the phase control with higher accuracy than that of the amplifying device of FIG. 5 is required for matching the phases, it is difficult to realize it, which has been a problem for improving the distortion compensation performance.